Disks to the Optical Analysis of Vibrating Bodies. 23 



the parallactic error. From this error may be explained the sur- 

 prising deception that the limbs of a large tuning-fork do not 

 appear to vibrate simultaneously towards one another. Again, 

 a vibrating steel wire fastened at one end, the length of which 

 coincides with the direction of motion of the holes when viewed 

 at some distance from the disk, does not appear simply bent, but 

 convoluted *. The parallactic error can only be eliminated by 

 making a = 0. Unfortunately great proximity of the eye to the 

 disk is, for various reasons, impracticable. By the disk G 

 (fig. 1) with the stationary apertures, the error is only in so far 

 diminished that the field of view is thereby diminished. With 

 the object-glass of a telescope, the centre of which in some sense 

 represents the cross-point of the eye, the approximation to the 

 disk can be as close as is desired. Hence upon the stand E t in 

 front of the disk G a small astronomical telescope F of only 1 J 

 to 2-fold magnifying-power may be screwed. The above fault 

 is thereby almost completely removed. On viewing the vibra- 

 tions of a tuning-fork through the telescope, both limbs vibrated 

 simultaneously and in opposite directions. Observed through 

 the telescope, the brightness is somewhat diminished; but in 

 this case, according to the degree of magnification, somewhat 

 larger holes may be used both in the disk B and in G. Of 

 course the aperture in the disk must in observing be accurately 

 centred in front of the objective. Observation through a tele- 

 scope has the advantage of a large field of view ; for the latter 

 only depends on the aperture of the object-glass. The telescope 

 is to be so arranged that it may be distinctly placed for aper- 

 tures of from 6 to 8 inches. 



The entire arrangement, as above described, I will briefly call 

 " Vibroscope." As to the experiments which may be made 

 with it, I may remark for the present the following : — 



(1) The vibrations of bars fastened at one end, and of tuning- 

 forks which are not too small, are easily investigated if they are 

 set up in front of a bright surface, or of a dark surface strongly 

 illuminated. Not only is the slow change of shape seen in a 

 surprisingly distinct manner, but by a tolerably constant rota- 

 tion of the vibroscope it is easy to estimate the velocity which the 

 vibrating body has in each part of its course. 



(2) The edges of a natural or artificial glottis show their 

 changes of form in the different phases of vibration very distinctly. 

 It cannot be doubted that by the aid of the laryngoscope and 

 intense illumination the vibroscope is suited for making physio- 

 logical studies on the living glottis during the production of sound. 



* These parallactic phenomena are just the essential points in Plateau's 

 anorthoscope ; for in it the rotating slits are obserTed from a tolerably great 

 distance. 



